Generally, this invention relates to thin film transistors especially as used in large area electronics such as information displays and light sensitive arrays for imaging. More particularly, this invention relates to thin film transistors fabricated on plastic substrates, in part by the novel low temperature processes of the invention, thus providing displays that are flexible, lighter in weight and more impact resistant than displays fabricated on traditional glass substrates.
Thin film transistors (TFTs) are used in many applications of large area electronics such as information displays and light sensitive arrays for imaging. In displays and imaging arrays, the TFT is used as a switch. The most common application is the active matrix liquid crystal display (AMLCD) which is the preferred display in laptop computers. In such a display, an array of display elements may be interconnected together with TFTs via horizontal and vertical bus bars. For example, the gates of one row of the plurality of TFTs in such displays are connected to a horizontal bus bar while the sources are connected to the vertical bus bars. When a voltage is applied to a predetermined horizontal bus bar and a predetermined vertical bus bar, the gate source and drain which form a particular TFT are energized. In the case of a liquid crystal display, that part of the liquid crystal which corresponds to the energized transistor becomes transparent and light from a source in back of the display is permitted to pass through.
More particularly, in an active matrix display, the TFT switches current on and off. When on, current flows to charge a capacitor associated with an individual pixel of the display to a desired voltage. When off, the capacitor is isolated and the selected charge remains until the next time the pixel is addressed. The voltage level determines the amount of light that is transmitted through the liquid crystal associated with the pixel (i.e., determines the grey level). In light imaging arrays (sensors), the TFT also switches current on and off. When off, charge is built up on a capacitor from a light sensitive diode (more light, more charge). When the TFT is on, the built-up charge is read out to the addressing circuit. The amount of the charge determines the intensity (i.e., grey level). In a different design of imaging array, the TFT is used to address a photosensitive resistor. Chemical sensors based on TFT""s have also been described.
Two common TFT structures are shown in FIG. 1. Referring to FIG. 1A, TFT 10 has the xe2x80x9cetch stopperxe2x80x9d structure and is made on glass plate 12. Gate metal 14 applies the gate voltage across gate dielectric 20. Current flows in channel layer 22 (amorphous or polycrystalline silicon) between source electrode 16 and drain electrode 18 through contact layers 26. Passivating insulator 24 isolates source 16 and drain 18 and prevents atmospheric degradation.
In FIG. 1B, TFT 30 has the xe2x80x9cback channel cutxe2x80x9d structure and is made on glass plate 32. Gate metal 34 applies the gate voltage across gate dielectric 40. Current flows in channel layer 42 (amorphous or polycrystalline silicon) between source electrode 36 and drain electrode 38 through contact layers 46. Passivating insulator 44 isolates source 36 and drain 38 and prevents atmospheric degradation.
Heretofore, the displays and photosensors described above have been fabricated on glass substrates and processing temperatures between about 250-400xc2x0 C. are required. For example, SiNitride layers 20, 24, 40 and 44 and a-Si:H layers 22, 42 are typically deposited by plasma enhanced chemical vapor deposition (PE CVD) and the deposition temperature typically exceeds 250xc2x0 C. during the PE CVD steps. Hence, only flat information systems that are relatively heavy and fragile have been possible. It would be beneficial if displays could be made lighter in weight, impact (shatter) resistant, and flexible. Curved displays would allow the user to experience a xe2x80x9cvirtual realityxe2x80x9d environment without wearing a display device on the user""s head, while curved photosensors would allow detection of a digital image from many directions at one time without moving the sensor array. Impact resistant and light weight displays are key enabling devices for truly portable information products such as laptop computers and personal navigation systems.
Larger and larger glass sheets are being introduced in the manufacture of active matrix liquid crystal displays (AMLCDs) in order to reduce the cost per display by making several displays at one time. The cost of manufacturing AMLCDs would be reduced significantly if thin plastic substrates and continuous roll-to-roll manufacturing methods could be used.
Fabrication of a TFT on a plastic substrate requires solutions to the following problems. Commercially available transparent plastics are dissolved, softened or attacked by many of the standard chemicals used to fabricate semiconductor devices. Thus, the plastic substrate must be made resistant to strong acids (including HF), bases, and hydrocarbon solvents. Inexpensive clear plastics soften or decompose at the standard processing temperatures for TFT fabrication which are typically between about 250-350xc2x0 C. Since all plastics have a coefficient of thermal expansion (CTE) typically 10 times that of glass, multilayer TFT structures built on plastic are prone to delamination at standard processing temperatures due to thermal expansion of the substrate.
Solutions to those problems have not heretofore been advanced. The present invention provides solutions to the above problems and discloses three exemplary TFT structures on plastic substrates.
A thin film transistor is described incorporating a gate electrode, a gate insulating layer, a semiconducting channel layer deposited on top of the gate insulating layer, an insulating encapsulation layer positioned on the channel layer, a source electrode, a drain electrode and a contact layer beneath each of the source and drain electrodes and in contact with at least the channel layer, all of which are situated on a plastic substrate. By enabling the use of plastics having low glass transition temperatures as substrates, the thin film transistors may be used in large area electronics such as information displays and light sensitive arrays for imaging which are flexible, lighter in weight and more impact resistant than displays fabricated on traditional glass substrates. The thin film transistors are useful in active matrix liquid crystal displays where the plastic substrates are transparent in the visible spectrum.
Enablement of the use of such plastics is by way of the use of polymeric encapsulation films to coat the surfaces of the plastic substrates prior to subsequent processing and the use of novel processes for the deposition of thin film transistor structures.